Project summary/Abstract Despite the combined antiretroviral therapy, around 50% of all treated patients still suffer cognitive impairments. HIV infection in the brain causes a spectrum of neurocognitive dysfunctions called HIV-associated neurocognitive disorders (HAND) with HIV-associated dementia (HAD), the most severe form. There is a high number of HIV-infected patients treated with antivirals that are affected by HAND, the risk of which increases with age and cardiovascular disease. HIV invades the brain early during the infection. The development of new strategies to avoid early and later brain infection would represent novel approaches to improve the HIV patient's quality of life, survival and everyday function. HIV infects target cells using the envelope glycoprotein trimers on the viral surface. The HIV trimer is formed by three glycoproteins (gp120) that interact directly with the CD4 receptor and then co-receptors allowing the virus entry into target cells. Macrophage tropic (M-tropic) viruses replicate in the brain because they have adapted to exploit low amounts of CD4 on the surface of macrophages and microglia. M-tropic viruses therefore carry more exposed CD4bs. Envs with a more open CD4bs may be more efficient at inducing antibodies to this site. There is a fundamental gap in knowledge of how each residue in gp120 regulates exposure of the CD4bs. Our hypothesis is that vaccines using trimeric Envs with an exposed CD4bs will induce protection against M- tropic viruses that infect the brain. Our objective here is to identify new gp120/gp41 mutations of diverse HIV Transmitted/Founder (T/F) clades that induce exposure of the CD4bs and to characterize the resulting Env trimers for their biological and structural properties. Guided by strong preliminary data, we propose: 1) to select T/F clade A, B and C trimers with exposed CD4bs, 2) to characterize these mutants by evaluating modifications in trimer structure and 3) to investigate the effect of this mutations on macrophage infection and co-receptor- use. This knowledge will be essential to understand how M-tropic viruses evolve to infect the brain and to develop vaccines that induce potent neutralizing antibodies against M-tropic viruses. This proposal thus represents a first and rational step to produce a vaccine with the potential to prevent the formation of, as well as targeting the HIV brain reservoir.